Fabrication of cermet film resistors to close tolerances



United States Patent 3,308,528 FABRICATION 0F CERMET FILM RESISTORS TOCLOSE TOLERANCES Robert L. Bullard, Wappiugers Falls, Robert A.Hasbrouck, Saugerties, Philip S. Schlemmer, Hyde Park, and Rudolf E.Thun, Poughkeepsie, N.Y., assignors to International Business MachinesCorporation, New York, N.Y., a corporation of New York Filed Nov. 6,1963, Ser. No. 321,928 2 Claims. (Cl. 29-155.7)

This invention relates to film electronics manufacture, and moreparticularly to the fabrication of film electronics panels having amultiplicity of cermet resistors wherein the ohmic value of theresistors is to be held to close tolerances. No. 182,818 filed Mar. 27,1962 (now U.S. Patent No. 3,261,082 issued July 19, 1966) having acommon assignee with this application, is directed to the generalproposition of tailoring a film resistor by subjecting it to anelectrical impulse. This application is directed to the peculiar problemof trimming cermet film resistors.

Cermet resistors, and more particularly chrom um-silicon monoxide thinfilm resistors have found considerable acceptance in the manufacture offilm electronic panels because such materials can be vacuum-deposited bymethods and equipment which are compatible with other steps in thefabrication of such panels, and may be made to yield a resistivity ofthe high ohms per square desired for compact circuitry panels.

The manufacture of optimum or even successful cermet resistors is notwithout problems, however. It is difficult or impossible to deposit amyriad of such resistors on a circuit panel in such manner that each hasprecisely its planned geometry, thickness and composition. Moreover, theopportunities for selection or mechanical trimming in order to bringresistors into tolerance are limited since, on the one hand, it is oftendesirable that as many circuits as possible be deposited in an integralunit on a single substrate, and, on the other hand, the individualresistors are likely to be so small and so closely located adjacent to,over, or under other elements as to be difficult to work upon by anymechanical or other grossly obtrusive means. Also, cermet resistors havethe peculiarity of being highly susceptible to absorption of elementsfrom the air when at an elevated temperature. Accordingly, it has becomea practice to encapsulate cermet film resistors completely in aprotective material such as silicon monoxide at the earliest practicablestep in the manufacturing process, even before the resistors areannealed.

The annealing process is provided to bring about changes in the internalstructure of the cermets whereby subsequent exposure to operating orother anticipated environmental temperatures will not cause significantalteration in the ohmic value of the resistors. nealing the ohmic valueof the resistors decreases toward their design value, but there islittle opportunity to treat separate resistors of a multiplicity on acommon substrate individually. Thus, the non-uniformity among theseveral resistors of a plurality on a common substrate occurring at thetime of deposition is perpetuated through the conventional annealingstep.

One might expect that since each resistor is encapsulated in siliconmonoxide, it would be impracticableto attempt to apply heat to resistorson an individual basis. Since silicon monoxide is a good heat insulator,it might be expected that by the time externally applied heat had verymuch effect on one resistor it would also have an undesirable effect onits neighbors. On the other hand, one might expect the same insulatingenvironment to result in confining the heat in a particular resistor insuch A prior, co-pending U.S. application, Ser.

During the an-' with the undercoat 12 to 3,308,528 Patented Mar. 14,1967 ICC manner that if heat were applied internally to such a resistorrapidly enough to avoid affecting its neighbors, hot spots within theresistor would result in its destruction and defeat for the method.

However, another characteristic of cermet resistors overcomes thisdifficulty. Since cermet resistors decrease in ohmic value uponapplication of heat, internally generated joule heat serves to effectthe greatest alteration on the most resistive spots in the resistor sothat hot spots tend to be eliminated and the application of such jouleheat equalized everywhere within the resistor. In accordance withpreferred embodiments of the present invention such joule heat isapplied to the resistors on an individual basis, by an apparatus whichmakes contact I with lands connected to opposite ends of a resistor forpassing current through the same and includes automatic means formeasuring the ohmic value of the resistor and terminating the joulecurrent promptly as the design value is reached. In this way, adjustmentof individual resistors can be undertaken with rapidity, both toexpedite the procedure and to guard against unwanted propagation of theheat to adjacent elements, and yet without danger of overshooting themark. The silicon monoxide encapsulation of each resistor provides anindividual oven or enclosure for the cermet element thereof which notonly provides a degree of heat isolation, but, more importantly affordsan air-free environment for the cermet during the joule heat treatment.

Accordingly, it is a primary object of this invention to provide anaccurate and facile means of producing cermet film resistors to veryclosely toleranced ohmic value.

It is another object of the invention to produce resistors as aforesaidin a manner which does not expose the resistors or associated oradjacent parts to seriously destructive or damaging influences.

It is yet another object of the invention to provide in a process asaforeclescribed, an improved means of adjusting the values of individualcermet resistors in a multiplicity of such resistors on a single,integral, film electronics panel.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following, more particulardescription of the preferred embodiment of the invention, as illustratedin the accompanying drawing.

FIG. 1 is a cross-sectional view of a cermet film resistor which may bebrought into closely toleranced ohmic value in accordance with thepresent invention;

FIG. 2 is a schematic diagram of a typical manufacturing process forpreparing the resistor of FIG. 1, in-

cluding the final step of adjusting or trimming its resistance inaccordance with the present invention; and

FIG. 3 is a schematic diagram of an apparatus which may be employed totreat and monitor the resistor of FIG. 1 in accordance with the processstep of FIG. 2.

The resistor shown in FIG. 1 is an example of the film electronicstructures the resistive element of which can be brought into closelytoleranced stable values in accordance with the invention. Typically,the structure may include a glass or other suitable substrate 10* whichforms a mechanical base upon which has been deposited a layer of siliconmonoxide 12 over which the cermet resistance material 14 is laid down.Copper or other suitably conductive terminals 16, 18 provide electricalconnection to the opposite ends of the resistance film 14, and anovercoat of silicon monoxide is provided which cooperates completeencapsulation of the resistance element 14. Thus, the cermet of theresistance element 14 is protected from the action of the atmosphere.

The techniques for preparation of film electronics elements such asshown in FIG. 1 are well known and it is also well known that theexercise of care in the deposition of the various layers can result inreasonably repeatable results, that is, resistors which have, after annealing, an ohmic value within five or ten percent of their planned ornominal value; The substrate 10 utilized may be glass polished to asurface roughness of fifty angstrom units and an over-all flatness of ofan inch across the working area of a substrate which will receive amultiplicity of such resistors, typically four by five inches indimension. All of the layers are deposited by evaporation in vacuo, andprocess parameters such as vacuum chamber pressure, substratetemperature, evaporator geometry, evaporation rate, film thickness andfilm composition are closely controlled. Thus, the chamber pressure maybe monitored with ionization gauges, and substrate temperatures measuredwith thermo-couples and controlled to within 10 C. by appropriate heatercontrol circuitry. A chromium underfiash 16a, 18a may be used prior todeposition of the copper terminal or land elements 16, 18 to assure goodadhesion, and evaporation rates for these metallic parts and the siliconmonoxide encapsulating layers 12, 20 may be controlled by an ion gaugerate monitor of the type having a heated collector for continuedaccuracy during its operation. The thickness of the resistance film 14itself can be estimated by utilization of a test slide within theevaporation chamber which is connected to a resistance meter.

Typically, the silicon monoxide is deposited at a substarte temperatureof 350 C., a pressure of 2 l0- torr, and a deposition of 25 angrstromunits per second. Chromium and copper for the elements 16, 1 8 may bedeposited at a substrate temperature of 160 C. and a pressure of X10torr, the chromium underfiash being sublimed from an open tungsten boatat 3 angstrom units per second and the copper deposited thereover from aradiantly heated baflled carbon crucible at 10 angstrom units persecond. The most critical element, the cermet film 14 itself, may bedeposited by flash evaporation of mixed chromium-silicon monoxidepowders at a substrate temperature of 160 C. and a pressure of 5-10 10-torr. As one example, the resistive film 14 may have a resistivity of300 to 400 ohms per square as deposited, which resistivity is reduced toabout 250 ohms'per square by annealing. Such a cemet film may be formedfrom 70 atomic percent chromium and 30 percent silicon monoxide and maybe in the order of 90 to 100 angstrom units thick. However, by varyingthe atomic percentages of chromium and silicon monoxide and thickness ofthe deposit, resistivities ranging from 10 ohms per square to megohmsper square may be fabricated.

Referring to FIG. 2, the foregoing typical deposition steps may besummarized as a first step 30 of depositing a 5,000 angstrom unitunderlayer 12 on a clean substrate 10, followed by a second step 32comprising the flash evaporation of premixed chromium-silicon monoxidepowders, 70 atomic percent chromium-30 percent silicon monoxide, to formthe cemet film 14 having an unannealed resistivity of 300 to 400 ohmsper square. In the next step 34, 5,000 angstrom units of copper with a300 angstrom unit underflash of chromium are deposited to form thelandor terminal elements 16, 18 and then in step 36, a 10,000 angstromoverlay 20 of silicon monoxide is deposited over the resistor material14. All of the steps to this point may be undertaken in a continuousprocess vacuum apparatus with masks brought into place at the variousstages of the operation to define the geometries of the several layers.In any case, it is preferable that the cermet element 14 be covered andencapsulated by the silicon monoxide layers 12, 20 before ever beingexposed to the air at an elevated temperature. It is a feature andadvantage of the present invention that no modification or interruptionof this encapsulation is necessary to achieve closely tolerancedresults.

In the preferred embodiment of the process of the invention, as in theprior art, the foregoing steps are followed by an oven annealing step 38which is carried out at about 350 to 475 C. in a reducing atmosphere offorming gas (10% hydrogen, argon) to protect the exposed copper landparts 16, 18 until a reduced, operationally stable resistance value isreached. This so-called annealing step brings about changes in thecermet as a function of the time and tempearture of the annealing. Theresulting gradual reduction of the resistivity of the cermet ismonitored. during the annealing, and the product pieces are taken out ofthe oven when a certain resistivity is reached. This heat treatment hasbeen found to result in resistors whose value is stable at operatingtemperatures below the annealing temperature.

In the prior art, the annealing step aims to achieve the design ornominal resistivity of, for example, 250 ohms per square :5%. Inaccordance with the present invention, the annealing is stopped somewhatshort of the goal, such as at the nominal resistivity +10/-0%, and thefinal adjustment to the design or nominal resistivity is made by thetrimming step 40 of the invention. Alternatively, the annealing step3 8can be omitted altogether, and, as indicated by the dotted line 42 inFIG. 2 the entire heat stabilizing and adjusting process can be carriedout by the joule heat procedure of the invention.

The trimming step 40 is carried out by passing current through theresistor so as to produce joule or re sistive heating therein in suchconcentration and for such a time as to bring the stabilized resistanceof the cermet element to, or closer to, the design or nominal objectivefor this value. This is accomplished by connecting the resistor intosuitable circuitry by use of its own terminals, without in any waydisrupting the silicon monoxide overcoat 20 which protects the cermetfilm 14 of the resistor device 22. Thus, the silicon monoxideundercoat-overcoat encapsulation 12, 20 continues to protect the cermetfilm 14 during joule heating of the same pursuant to the trimming step.

This procedure may be carried out with facility by use of an apparatusof the kind shown in FIG. 3. -It will be understood that the resistor 22to be trimmed may be, and usually is, one of a multiplicity on a singlesubstrate 10, the substrate 10 being for this purpose extensive comparedto the area covered by a single resistor 22, as indicated by the brokenline portions 42, 44 thereof. Thus, the apparatus of FIG. 3 may beduplicated a plurality of times in a jig adapted to treat the number ofresistors 22 on a single substrate at the same time. It is neverthelessdesirable that the treatment of each resistor be an individual matter,for enabling close, individual trimming of each resistor. Thus, thecircuit of FIG. 3 is typical of that which may be employed for treatmentof a single resistor or with others l ke it in a jig, for simultaneoustreatment of a plurality of resistors.

In the apparatus of FIG. 3, the power for joule heating is applied tothe resistor through contacts 46, 48 which bear against the exposedsurfaces of the resistor terminals 16, 18 for connecting them to anelectrical power source preferably in the form of a constant currentgenerator 50. As the cermet film 14 within the resistor 22 is subjectedto the resulting joule heat, during the trimming operation, itsresistivity will decrease. Therefore, a constant current generator is apreferred form of power supply since the rate of resistance heating willdecrease somewhat as the nominal or final resistance value isapproached, rather than the converse which would occur in the case of aconstant voltage supply. For minimizing inaccuracies due to contactresistance, it is preferred that the resistance value of the.

element 22 being trimmed be monitored through a separate pair orcontainers 52, 54 which also contact the resistor terminal lands 16, 18.'The several cont actors 46, 48, 50, 52 may be in the form of stainlesssteel rods the ends of which have been lapped to a smooth, transverse,planar shape, and are spring-mounted so as to press firmly against theterminal lands 16, 18 of the resistor.

and any slight discoloration of the terminal lands 16, 18 can be cleanedolT afterward with a suitable etch. In a few cases, as shown in some ofthe examples in Table I below, it is necessary to go to higher heatapplication The resistivity monitoring contactors 52, 54 are con- 5rates in order to move the value of the resistance. nected in a circuitwhich may include a standard voltage These conditions are encountered inthe cermet mixes source 56 and a comparator circuit 53. The polarityhaving a very high proportion of chromium such as the relation of theoutput of the standard voltage source 90 percent chromium cermet or incases where the re- 56 is made the same as that of the current generatorsistivity as deposited has required long, high temperature 50, asindicated on the drawing, and the function of the 10 annealing.comparator circuit 58 is to yield an output whenever Table I is acorrelation of data, showing the interrelathe potential at contactor 52drops in magnitude below tion of certain parameters in the carrying outof the that at output line 60 of the standard voltage source. process inaccordance with the invention. In the cases Any convenient circuit canbe used for the comparaof sample #3, 4 and 7, the resistors were left inthe hot tor 58; as a simple example a high gain, saturating amplivacuumchamber after deposition of the SiO overcoat unfier could be utilized,as will be obvious to those skilled til they had reached a reduced,substantially stabilized in the electrical arts. The output of thecomparator 58 value. Thus, the as deposited resistivity was notmeascontrols a shunt 62, 64 across the current generator 50. ured, andthere is no oven anneal data, per se.

TABLE 1 Deposition Anneal Trim No. 7 Final Cr/SiO Area, Length/ Nom.Act. Time, Hrs, Temp, R./Sq., Nom R./Sq Mix, at. in. Width R./Sq.,R./Sq., Min. 0. ohms I, ma. Power, percent Ratio ohms ohms W./in 2 1 Noreading.

Thus the output device of the comparator may be in the form of thesolenoid 66 of a relay having a normally open contactor 68 in the shuntcircuit 62, =64. The same apparatus may be utilized. to trim resistorsof various-design or nominal resistivity, and accordingly the standardor reference voltage supply 56 is preferably adjustable. So also, sinceresistors may vary not only in ohmic value but in other parameters suchas area, it is desirable that the constant current generator 50 beadjustable for enabling selection land/or approximate standardization ofthe power dissipation rate to be applied to different resistors.

Thus, an apparatus is provided whereby a desired rate of joule heatingmay be applied to the resistor 22 to be trimmed, and which terminatespromptly the application of that power when the potential dfferen-ceacross the resistor falls to that across the standard voltage of themonitoring apparatus, in other Words, when the resistance of the device22 on being trimmed falls to the desired value.

A number of parameters enter into selection of the optimum rate of jouleheating to be applied to a particular resistor. The general objective isto raise the temperature of the cermet film of the resistor well aboveits annealing temperature so as to bring its resistivity to the desiredvalue in a short period of time, both for speeding the operation and tominimize unwanted conduction of heat to adjacent elements. At the sametime, it is desired that the application of heat not be so great as toseriously burn the terminals 16, 18 or damage surrounding elements byheat, shock or otherwise. As a general rule, it has been found that aheating rate of about one killowatt per square inch of cermet beingtreated is usually satisfactory for producing marked and rapidadjustment of the resistivity of most cermets without serious damage.The substrate glass is usually of the heat shock resistant kind,

Since the cut-off of the trimming current is automatic, the duration ofthe trimming current was not measured. It is ordinarily less than twoseconds. In Example #1, however, the high-chromium cermet did not triminto a closer approach to its nominal value of 40 ohms per square withapplication of three kilowatts per square inch for several minutes, andtherefore trimming was discontinued manually to avoid destruction of theresistor. This example is included to show the remarkable heatdissipating capability of these resistors.

An experiment was conducted for the purpose of estimating thetime-temperature relation which exists during the trimming of cermetresistors in accordance with the invention. The results were consistantwith the conclusion that the mechanism of the trimming step is that of afurther, higher temperature annealingwhat one might call joule annealingas opposed to oven annealing as above described with respect to step 38,FIG. 2, or the variant of that oven annealing which occurs when theworkpiece is kept hot in the deposition vacuum chamber as in the case ofExamples 3, 4 and 7 of Table I.

This experiment is reported in TableII below. All of the sampleresistors were on a single substrate and were of 50/50 at. percentCr-SiO cermet of nominal resistivity of 1000 ohms per square. They wereundercoated with 10,000 A. of SiO and overcoated with 10,000 A. of SiO.They had been oven-annealed for a total of 31 /2 hours at about 450 to475 C. A bit of the indicator was placed on the SiO overcoat, aboutmidway over the cermet element, and trimming current was applied to theresistor until the indicator was observed to melt. The trimming currentwas calculated to produce joule heating of one kilowatt per square inchat the nominal resistivity; therefore the actual power applied wassomewhat above that amount.

TABLE II Act. Resistance,

ohms Molt Sample Nominal Trim. Time to melt Indicator Temp,

Resistance indicator C.

Before After Trim Trim 4, 000 4, 635 4, 213 Al 660 4, 000 4, 722 4, 519Al 660 4,000 4, 622 4, 611 CdCiTZVgTIQO 568 1,000 1,173 1,159 CdC1-2%I'I2O .m 568 4, 000 4, 730 4, 727 Pb 327 1, 000 1, 167 1, 167 327 1,000 1,164 1,130 419 4, 000 4, 681 i 4, 610 419 4, 000 4, 717 4. 713 4221, 000 1, 059 l. 057 422 Discounting Sample #H as experimental error, itwill be seen that the outer surface of the SiO overcoat where theindicator was placed rose to the vicinity of the oven annealingtemperature in well under two seconds. In the case of Samples #C and D,a rise to about 100 C. over the oven annealing temperature was indicatedin only about a quarter second, perhaps because the cadmium chlorideindicator was in the form of fiat crystals making better thermal contactthan the aluminum or lead, which were in chip form, or the Zinc orcupro-us chloride which were in powdered form. Of course, the cermctitself would in every case be hotter than the indicator, since the SiOtherebetween is an insulator. Thus, it is believed clear that thetrimming is brought about by an annealing action in which the ce-rmet issubjected to a temperature higher than its previous annealingexperience.

From the foregoing, it will be seen that the invention provides a meansof treating cermet resistors in a most flexible manner. Not only caneach resistor of a multiplicity on a substrate be treated separatelyfrom its neighbors, but also the scope and precision of the treatment issuch as to make up for wide variation in the process steps preceding itand to enable relaxation of some of the controls of those steps. Thus,instead of attempting to carry out the oven annealing steps of the priorart to maximum accuracy, it is feasible to stop the oven annealingconsiderably before the nominal resistivity is reached, leaving all ofthe final accuracy to the trimming or joule annealing step of theinvention.

Table I above was assembled from data which show application of theinvention under a wide variety of con ditions. Table III, below, showsmore typical results which illustrate the uniformity and reproducibilityof the process.

Table III relates to a single panel having 48 resistors of 50/50 atomicpercent chromium-silicon monoxide cermct which had been oven annealed at350390 C. for onehalf hour. The cermet film had a nominal resistivity ofabout300 ohms per square and the resistors were of 2:1 length-widthratio, or 600 ohm nominal resistance. The variation of the 48 untrimmedresistors was from 616 ohms to 702 ohms, or 86 ohms, while the valuesfor the :same resistors after trimming were found to be 600, +1/ -2 ohmsor a total variation of 0.5

TABLE III It is possible where desired to omit the oven annealing as aseparate procedure and substitute the relatively crude but sometimesmore convenient technique of utilizing so called vacuum annealing, forexample as in the cases of Samples #3, 4 and 7 in Table I. It will berecalled that the substrate temperature during the final deposition ofsilicon monoxide is about 375 C., so that retention of the workpiece inthe hot vacuum chamber after the deposition of that layer for thirtyminutes or so produces a degree of annealing which can be finished bythe further annealing of the trimming step of the invention. As stillanother option, the entire annealing process can be performed by thejoule heating of the invention without any substantial anneal steptherebefore, as indicated by line 42, FIG. 2.

While the invention has been particularly shown and described withreference'to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A method of fabricating a cermct thin film resistor to apredetermined design resistivity comprising providing a substrate andforming thereon conductive terminal elements and a chromium-siliconmonoxide cermct film in lapped relation to form an electrical circuitpath between said elements through said film, said film having aresistivity greater than said design resistivity and being encapsulatedcompletely in silicon monoxide, i y then annealing said film bysubjecting the same to a temperature of about 400 C. in an oven untilthe resistivity of said film approaches said design resistivity, I l andfinally heating the cermct film circuit element to a further annealingtemperature by passing electrical current through said element until theresistivity of said film reaches said design resistivity. 2. A method offabricating a cermct thin film resistor to a predetermined designresistivity comprising providing a substrate and forming thereonconductive terminal elements and a chromium-silicon monoxide cermct filmin lapped relation to form an electrical circuit path between saidelements through said film, said film being in the order of one hundredangstrom units thick and having a resistivity greater than said designresistivity and being encapsulated completely in silicon monoxide, I

then annealing said film by subjecting the same to a temperature ofabout 400 C. in an oven until the resistivity of said film approachessaid design resistivy,

9 10 and finally heating the cermet film element to a Iuragag; gatfi ggggigther annealing temperature, whlle monitoring the re- 2,786,9253/1957 Kahan 338 308 X SlStaHCB of sand element, by passing electricalcur- 2 994 847 8/1961 Vodar 338 208 rent through said element sufficientto produce joule 5 33055019 10 19 3 Davis 338-308 X heating of about onekilowatt per square inch in said element until the resistivity of saidfilm reaches said design resistivity.

JOHN F. CAMPBELL, Primary Examiner. WHITMORE A. WILTZ, Examiner. W. I.BROOKS, Assistant Examiner.

1. A METHOD OF FABRICATING A CERMET THIN FILM RESISTOR TO APREDETERMINED DESIGN RESISTIVITY COMPRISING PROVIDING A SUBSTRATE ANDFORMING THEREON CONDUCTIVE TERMINAL ELEMENTS AND A CHROMIUM-SILICONMONOXIDE CERMET FILM IN LAPPED RELATION TO FORM AN ELECTRICAL CIRCUITPATH BETWEEN SAID ELEMENTS THROUGH SAID FILM, SAID FILM HAVING ARESISTIVITY GREATER THAN SAID DESIGN RESISTIVITY AND BEING ENCAPSULATEDCOMPLETELY IN SILICON MONOXIDE, THEN ANNEALING SAID FILM BY SUBJECTINGTHE SAME TO A TEMPERATURE OF ABOUT 400*C. IN AN OVEN UNTIL THERESISTIVITY OF SAID FILM APPROACHES SAID DESIGN RESISTIVITY, AND FINALLYHEATING THE CERMET FILM CIRCUIT ELEMENT TO A FURTHER ANNEALINGTEMPERATURE BY PASSING ELECTRICAL CURRENT THROUGH SAID ELEMENT UNTIL THERESISTIVITY OF SAID FILM REACHES SAID DESIGN RESISTIVITY.